The field of cultivating plants has spurred technological advances from the plow, to artificial irrigation, to hybridization and now to advances in the application of DNA research. In the area of subtle influences that alter a plants environment, some have experimented with “talking to their plants” and playing Mozart for them. While neither of those techniques has found widespread use, there is a growing body of serious research regarding the effects of sound and vibrations on plant growth. Like all living organisms, plants have highly complex sensory networks for monitoring their surroundings, and are known to modify their growth and development to suit their environment. For example, plants exposed to a variety of mechanical perturbations, such as wind or touch, undergo physiological and developmental changes that enhance resistance to subsequent mechanical stress. Developmental changes in response to mechano-stimulation are collectively known as thigmomorphogenesis.
The short paper “Biochemical and physiological changes in plants as a result of different sonic exposures” by Yu-Chuan Qin, Won-Chu Lee, Young-Cheol Choi and Tae-Wan Kim that was published in Elsevier's Ultrasonics journal (41 (2003) 407-41) investigates the biochemical mechanisms that might be involved in some of these phenomena. Chinese cabbage and cucumbers at two growth stages were the researchers' subjects. For each plant type three groups were constituted. Besides a control group that was not subject to any artificial acoustic treatment, one group was exposed to steady ultrasonic (US) waves of 20 k Hz, while the other was exposed to so-called “green music” (GM) consisting of a combination of classical music and natural sounds including bird songs. Both O2 intake and polyamines content were measured. In brief, they found Chinese cabbage reacting more positively to the GM and the cucumbers to the US. However, for each quantity measured, either one or the other or both of the sonically exposed plants had greater readings than those of the control plants. That paper's charts of the polyamines content measurements are reproduced as FIGS. 1A and 1B. The caption of the Chinese cabbage growth graph in that paper is:                “Polyamine content (nmol/gFW) of Chinese cabbage seedlings: (A) 15 d and (B) mature plant (70 d) as a result of different acoustic exposures. Error bars represent the standard deviations of the means of polyamine contents.” And the caption in that paper of the cucumber data is: “Polyamine content (nmol/gFW) of cucumber seedlings: (A) 15 d and (B) mature plant (70 d) as a result of different acoustic treatments. Error bars represent the standard deviations of the means of polyamine contents.”        
Studies have also focused on specific frequencies' effects, for example “Plant gene responses to frequency-specific sound signals”, Mi-Jeong Jeong, Chang-Ki Shim, Jin-Ohk Lee, Hawk-Bin Kwon, Yang-Han Kim, Seong-Kon Lee, Myeong-Ok Byun and Soo-Chul Park. (Mol Breeding (2008) 21:217-226) published Springer's Molecular Breeding journal. They demonstrated sound affecting plant growth through mRNA expression analyses.
Others have looked at the issue of the effect of vibration on plant growth. One relevant article is: “Growth Promotion by Vibration at 50 Hz in Rice and Cucumber Seedlings”, Hideyuki Takahashi, Hiroshi Suge and Tadashi Kato. (Plant Cell Physiol. 32(5): 729-732 (1991)). They looked at the effect of 50 Hz vibration and mention that a motivation of their study was the issue that motors and other mechanical apparatus in a green house might produce sounds with unintended and unexpected effects on plants.
FIG. 3 shows a reproduction of that paper's “FIG. 1.” Its caption is:                “Germination of rice and cucumber seeds as affected by vibration at 50 Hz Data is shown as the percentage of germinated seeds in a time-course study. Top (A), rice seeds under submerged conditions; middle (B), rice seeds on filter paper; bottom C), cucumber seeds on filter paper. Open (◯) and closed (●) circles indicate the control and the vibrated seeds, respectively. One hundred seeds were used for each treatment.”        
U.S. Pat. No. 7,600,343 dated Oct. 13, 2009 by Reiner Schultheiss, et al, discusses the effect of shock waves on plant growth.
However, previous attempts to improve plant growth along the lines of the research above have not made it into routine, large-scale, commercial use. Systems and methods are needed which can improve plant growth in ways compatible with our current environmental imperatives that are also inexpensive to deploy and maintain. Preferably, solutions would avoid chemical fertilizers and chemical pesticides and be simple to deploy in both the developed world and the developing world.